The invention relates to fluid injection devices, and more particularly, to fluid injection devices preventing activation of a bipolar junction transistor (BJT) therein.
Typically, fluid injection devices are employed in inkjet printers, fuel injectors, biomedical chips and other devices. Among inkjet printers presently known and used, injection by thermally driven bubbles has been most successful due to reliability, simplicity and relatively low cost.
FIG. 1 is a cross section of a conventional monolithic fluid injector disclosed in U.S. Pat. No. 6,471,338, the entirety of which is hereby incorporated by reference. A conventional monolithic fluid injector 10 is fabricated by micro-electro-mechanical system (MEMS) and metal oxide semiconductor field effect transistor (MOSFET) processes. The conventional monolithic fluid injector comprises a base such as a silicon substrate 38 with a field oxide layer 50 thereon. A structural layer 42 is formed on the field oxide layer 50. A fluid chamber 14 is formed between the silicon substrate 38, the field oxide 50, and the structural layer 42. The fluid chamber 14 connects a fluid reservoir (not shown) via a channel 16. A first heater 20 and a second heater 22 are formed on the structural layer 42. A dielectric layer 45 is disposed overlying the structural layer 42 defining a nozzle 17. The nozzle 17 adjacent to the first and the second heaters 20, 22 connects the fluid chamber 14. The first and the second heaters 20 electrically connect a driver via a signal transmitting circuit 44. The driver is a MOSFET comprising a drain 107, a gate 105 with a gate dielectric layer 52 between the 105 and the base 38, and a source 106, wherein the drain 107 electrically connects the signal transmitting circuit 44. A passivation 46 is disposed on the fluid injection device and the driver.
As the development of fabrication processes has progressed, fluid injection devices with high density nozzles and multiple activation methods thereof to increase printing quality and speed have been introduced. A driver integrated with conventional fluid injection devices comprises a MOSFET device. When multiple nozzles are activated simultaneously, parasitic bipolar junction transistors (BJT) can be triggered, causing abnormal injection. The abnormal injection not only reduces printing quality, but also overheats the heaters, reducing the lifetime of the fluid injection device.
Accordingly, fluid injection devices with high density nozzles and multiple activation methods which do not activate parasitic bipolar junction transistors (BJTs) are desirable.